The purpose of this research is to test the hypothesis that an endogenous substance exists which has cardiac glycoside-like effects (endodigin) and to isolate an characterize such a principle(s). In order for a putative principle to qualify asendodigin, the following criteria should be fulfilled: 1) Endodigin should bend to the ouabain receptor and in so doing inhibit (3H)ouabain binding to Na+, K+-ATPase, 2) Endodigin should "chase" the label from the enzyme-(3H)ouabain complex at the same rate as unlabeled ouabain, 3) Endodigin should inhibit Na+, K+-ATPpase specifically, and 4) Endodigin should cause positive inotropism. The approach to be taken will be to obtain a water-acetone extract from rat brain which specifically inhibits Na+,K+-ATPase. Recently, several reports have appeared suggesting the presence of a digitalis-like substance in acid-acetone extracts of brain. We have demonstrated that in preparing an acid-acetone extract, lysophospholipids and peroxidized lipids, both of which inhibit Na+,K+-ATPase are artifactually produced. Preliminary evidence suggests that a water-acetone extract of rat brain contains a principle which specifically inhibits Na+,K+-ATPase. The brain extract which specifically inhibits Na+,K+-ATPase will be subjected to column chromatography for separation of components and reverse phase high-performance liquid chromatography for preliminary purification. The fraction demonstrating specific inhibition of Na+,K+-ATPase will be tested for its effect on ouabain binding and inotropism. Once the endodigin is isolated, an effort will be made to manipulate the amount of principle in vivo by volume expansion and hypothalamic lesions. The characterization of the principal will provide the basis for identification, synthesis and use in pharmacological therapeutics. It is conceivable, base on analogy to enkephalin/opiate receptor, that a patient in congestive heart failure may be treated by stimulating his own production of endodigin or be the administration of wynthesized rather than receiving the currently available cardiac glycosides.